Dielectric Insulation Medium

ABSTRACT

A dielectric insulation medium. The insulation medium is characterized in that it comprises a fluoroketone having from 4 to 12 carbon atoms.

FIELD OF THE INVENTION

The present invention relates to a dielectric insulation medium and tothe use of a fluoroketone in such an insulation medium. The inventionfurther relates to an apparatus for the generation, the distribution orthe usage of electrical energy, and a dimensioning method.

BACKGROUND OF THE INVENTION

Dielectric insulation media in liquid or gaseous state areconventionally applied for the insulation of an electrical active partin a wide variety of electrical apparatuses, such as switchgears ortransformers.

In medium or high voltage metal-encapsulated switchgears, for example,the electrical active part is arranged in a gas-tight housing, whichdefines an insulating space, said insulation space comprising aninsulation gas usually with several bar pressure and separating thehousing from the electrical active part without letting electricalcurrent to pass through. Thus, metal-encapsulated switchgears allow fora much more space-saving construction than switchgears which are mountedoutdoors and insulated solely by ambient air. For interrupting thecurrent in a high voltage switchgear, the insulating gas furtherfunctions as an arc extinction gas.

Conventionally used insulation gases with high insulation and switchingperformance have some environmental impact when released to theatmosphere. So far, the high global warming potential (GWP) of theseinsulation gases has been coped with by strict gas leakage control ingas-insulated apparatuses and by very careful gas handling. Conventionalenvironmental-friendly insulation gases like e.g. dry air or CO₂ have aquite low insulation performance, so that gas pressure and/or insulationdistances would have to be increased.

For the reasons mentioned above, efforts have been made in the past toreplace these conventional insulation gases by suitable substitutes.

For example, WO 2008/073790 discloses a dielectric gaseous compoundwhich—among other characteristics—has a boiling point in the rangebetween about −20° C. to about −273° C., which is low, preferablynon-ozone depleting and which has a GWP less than about 22,200.Specifically, WO 2008/073790 discloses a number of different compoundswhich do not fall within a generic chemical definition.

Further, U.S. Pat. No. 4,175,048 relates to a gaseous insulatorcomprising a compound selected from the group of perfluorocyclohexeneand hexafluoroazomethane, and EP-A-0670294 discloses the use ofperfluoropropane as a dielectric gas.

EP-A-1933432 refers to trifluoroiodomethane (CF₃I) and its use as aninsulating gas in a gas-insulated switchgear. In this regard, thedocument mentions both the dielectric strength and the interruptingperformance to be important requirements for an insulating gas. CF₃I hasaccording to EP-A-1933432 a GWP of 5 and is thus considered to causerelatively low environmental load. However, because of the relative highboiling point of CF₃I (−22° C.) gas mixtures with CO₂ are taught.Additionally, pure CF₃I-gas has about the same insulation performance asconventional insulation media having a high insulation and switchingperformance, so that the proposed gas mixtures have around 80% of thespecific insulation performance of a pure conventional insulation mediumwhich would have to become compensated by increased filling pressureand/or larger insulation distance.

Therefore there is an ongoing need for an insulation medium which causeseven less environmental load than CF₃I and does not require an increaseof the gas pressure and/or the insulation distances above today usualvalues.

SUMMARY OF THE INVENTION

In view of this, the objective of the present invention is thus toprovide an insulating medium having a reduced GWP, but having at thesame time comparable or even improved insulation properties incomparison to the known insulation media without an increase of the gaspressure and/or the insulation distances above today applied values.

This objective is achieved by the insulation medium according to theindependent claims. Preferred embodiments of the invention are given inthe dependent claims.

The invention is based on the surprising finding that by using afluoroketone having from 4 to 12 carbon atoms an insulation mediumhaving high insulation capabilities, in particular a high dielectricstrength (or breakdown field strength), and at the same time anextremely low global warming potential (GWP) can be obtained.

In general, the fluoroketone according to the present invention has thegeneral structure

R1—CO—R2

wherein R1 and R2 are at least partially fluorinated chains, said chainsbeing independently from each other linear or branched and having from 1to 10 carbon atoms. The definition encompasses both perfluorinatedketones as well as hydrofluorinated ketones.

Generally, the fluoroketone used according to the present invention hasa boiling point of at least −5° C. at ambient pressure which is in clearcontrast to the teaching of the state of the art and in particular of WO2008/073790 which teaches a boiling point of −20° C. or lower to be anessential feature of a feasible dielectric compound.

Preferably, the fluoroketone has from 4 to 10 carbon atoms, morepreferably from 4 to 8 carbon atoms, and most preferably 6 carbon atoms(also referred to as a C6-fluoroketone). As mentioned above, saidC6-fluoroketone can be a perfluorinated ketone (having the molecularformula C₆F₁₂O) or a hydrofluorinated ketone.

In use, the insulation medium can be both in liquid and gaseous state.In particular, the insulation medium can be a two-phase systemcomprising the fluoroketone both in liquid and gaseous state. Moreparticularly, the insulation medium can be an aerosol comprisingdroplets of the fluoroketone dispersed in a gas phase comprisingfluoroketone in gaseous state.

For many applications, it is preferred that the insulation mediumcomprises an insulation gas comprising the fluoroketone at operationalconditions. This is in particular the case for an insulation medium usedfor high voltage switching in a corresponding switchgear.

If an insulation gas is used, it can either be a gas mixture, whichapart from the fluoroketone preferably comprises air or at least one aircomponent, in particular selected from the group consisting of carbondioxide (CO₂), oxygen (O₂) and nitrogen (N₂), as buffer or carrier gas.Alternatively, the insulation gas can substantially consist offluoroketone.

The insulation properties of the insulation gas, and in particular itsbreakdown field strength, can be controlled by the temperature, pressureand/or composition of the insulation medium. If a two-phase systemcomprising the fluoroketone both in liquid and gaseous state is used, anincrease of the temperature does not only result in an increase of theabsolute pressure, but also in an increase of the fluoroketone'sconcentration in the insulation gas due to a higher vapour pressure.

It has been found that for many applications of the insulation gas, suchas applications in the medium voltage range, a sufficient molar ratio,i.e. the ratio between the number of molecules of the fluoroketone tothe number of molecules of the remaining components of the medium(generally the carrier or buffer gas), and thus also a sufficientbreakdown field strength can be achieved even at very low operationaltemperatures e.g. of down to about −30° C. or even −40° C., withoutadditional measures such as external heating or vaporization.

If a higher concentration of the fluoroketone in the insulation gas isdesired to increase the breakdown field strength, which may inparticular be the case in high voltage applications, the pressure, thecomposition and/or the temperature of the insulation medium can beadapted accordingly. A way how to deduce the parameters required toobtain a desired breakdown field strength will be further exemplified inthe context of the Figures below.

The dielectric insulation medium of the present invention can be used inany apparatus for the generation, the distribution or the usage ofelectrical energy, particularly in a switchgear or a part and/orcomponent thereof.

For high voltage switching, for example, the interrupting capability (orarc extinction capability) of the insulation medium is of particularimportance. It has surprisingly been found that the medium according tothe present invention not only has a comparable or even improvedinsulating capability compared to the above mentioned conventionalinsulation media, but also a sufficient arc extinction capability.Without any intention to be bound by the theory it is assumed that thisarc extinction capability can at least partially be attributed to therecombination of the dissociation products of the fluoroketone insidethe arcing region mainly to tetrafluoromethane (CF₄) which is well knownto be a highly potent arc extinction medium.

Another important aspect during arc interruption is the temperatureincrease of the switching gas in the whole vessel which may lead toinsulation failures to the grounded vessel even after successful arcinterruption inside the switching gap, especially after heavy faultinterruption in metal-encapsulated circuit breakers. Due to thedecomposition of fluoroketones at moderate temperatures (e.g. around550° C. to 570° C. for C6-fluoroketone) to lower fluorocarbons, theinjected heat energy in the exhaust volumes does not lead totemperatures above these dissociation temperatures, until allfluoroketone is dissociated. If sufficient fluoroketone is provided, theexhaust gas temperature therefore cannot exceed the above mentionedtemperatures leading to a good insulation performance also shortly afterthe interruption of a heavy fault current in a metal-encapsulatedhigh-voltage circuit breaker.

Among the most preferred fluoroketones having 6 carbon atoms,dodecafluoro-2-methylpentan-3-one has been found to be particularlypreferred for its high insulating properties and its extremely low GWP.

Dodecafluoro-2-methylpentan-3-one (also named1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone,perfluoro-2-methyl-3-pentanone or CF₃CF₂C(O)CF(CF₃)₂) has previouslyonly been considered useful for completely different applications,namely the processing of molten reactive metals (as referred to in WO2004/090177), for the cleaning of a vapour reactor (as referred to in WO02/086191) and in fire extinction systems, or in liquid form for coolingof electronic systems, or for the Rankine-process in small power plants(as referred to in EP-A-1764487).

Dodecafluoro-2-methylpentan-3-one is clear, colorless and almostodourless. Its structural formula is given in the following:

Dodecafluoro-2-methylpentan-3-one has an average lifetime in theatmosphere of about 5 days and its GWP is only about 1. In addition, itsozone depletion potential (ODP) is zero. Thus, the environmental load ismuch lower than the one of conventional insulation gases.

In addition, dodecafluoro-2-methylpentan-3-one is non-toxic and offersoutstanding margins of human safety. This is in contrast tofluoroketones having less than 4 carbon atoms, such as hexafluoroacetone(or hexafluoropropanone), which are generally toxic and very reactive.

Dodecafluoro-2-methylpentan-3-one has a boiling point of 49.2° C. at 1bar. Its vapour pressure, i.e. the pressure of the vapor in equilibriumwith its non-vapor phases, is about 40 kPa at 25° C. Given the highvapour pressure of dodecafluoro-2-methylpentan-3-one, an insulation gashaving a breakdown field strength sufficient for many applications, inparticular in the medium voltage range, can in general also be achievedat very low temperatures down to −30° C.

If the insulation medium is an insulation gas, as it is for examplepreferably the case in a circuit breaker of a high voltage switchgear,dodecafluoro-2-methylpentan-3-one can either be provided in a gasmixture, which preferably further comprises air or at least one aircomponent functioning as a carrier or buffer gas. Alternatively, theinsulating gas can substantially consist ofdodecafluoro-2-methylpentan-3-one.

Based on the finding that at a temperature of 550° C. or higher,dodecafluoro-2-methylpentan-3-one is decomposed to very reactivefluorocarbon compounds having a lower number of carbon atoms, it ispreferred that the insulating gas comprises sufficient oxygen (O₂) withwhich the fluorocarbon compounds formed can react to form inertcompounds, such as e.g. CO₂.

According to a particularly preferred embodiment of the presentinvention, the molar ratio of the fluoroketone, in particular ofdodecafluoro-2-methylpentan-3-one, in the insulation gas is at least 1%,preferably at least 2%, more preferably at least 5%, more preferably atleast 10%, most preferably at least 15%. These preferred molar ratiosrefer to a given standard or prescribed operating condition. Underdeviating conditions, the molar ratio may still vary from thesepreferred values.

The significance of an insulating medium comprisingdodecafluoro-2-methylpentan-3-one in a molar ratio of at least 1%, or 2%respectively, is based on the finding that an insulation gas having thismolar ratio can also be obtained at very low temperature conditions downto −30° C. for 2% and down to −40° C. for 1% and that this insulationgas has a sufficient dielectric strength for e.g. medium voltageapparatuses, such as medium voltage gas-insulated switchgears, which areoperated at an insulation gas pressure of around 1 bar and in particularbelow 1.5 bar.

As will be further illustrated by way of the examples, the insulatingcapability of an insulating gas having a molar ratio ofdodecafluoro-2-methylpentan-3-one of at least 15% is (at 1 bar) evenhigher than that of conventional insulating gases. This embodiment isthus particularly preferred.

It is a further objective of the present invention to provide improveddielectric insulation and improved electrical apparatuses comprising theinsulation medium described above. This objective is achieved accordingto the claims by the use of the above-described fluoroketone fordielectric insulation and, in particular, for arc extinction, andaccording to the claims by an apparatus comprising the above-describedfluoroketone. Preferred embodiments are disclosed and claimed in thedependent claims.

Therefore, in addition to the insulation medium described above, thepresent invention further relates to an apparatus for the generation,the distribution and the usage of electrical energy, said apparatuscomprising a housing defining an insulating space and an electricalactive part arranged in the insulating space. This insulating spacecomprises the insulation medium described above.

The term “or” in the expression “apparatus for the generation, thedistribution or the usage of electrical energy” is in this context notto be understood as excluding combinations but is to be read as“and/or”.

Also, the term “electrical active part” in this context is to beinterpreted broadly including a conductor, a conductor arrangement, aswitch, a conductive component, a surge arrester, and the like.

In particular, the apparatus of the present invention includes aswitchgear, in particular an air-insulated or gas-insulated metal (orotherwise)-encapsulated switchgear, or a part and/or component thereof,in particular a bus bar, a bushing, a cable, a gas-insulated cable, acable joint, a current transformer, a voltage transformer, a surgearrester, an earthing switch, a disconnector, a load-break switch,and/or a circuit breaker.

Switchgears, in particular gas-insulated switchgears (GIS), are wellknown to a person skilled in the art. An example of a switchgear forwhich the present invention is particularly well suited is for exampleshown in EP-A-1933432, paragraphs [0011] to [0015], the disclosure ofwhich is incorporated herewith by reference.

It is further preferred that the apparatus is a switch, in particular anearthing switch (e.g. a fast acting earthing switch), a disconnector, aload-break switch or a circuit breaker, in particular a medium-voltagecircuit breaker, a generator circuit breaker and/or a high-voltagecircuit breaker.

According to another preferred embodiment, the apparatus can be atransformer, in particular a distribution transformer or a powertransformer.

According to still other embodiments, the apparatus can also be, e.g.,an electrical rotating machine, a generator, a motor, a drive, asemiconducting device, a computing machine, a power electronics device,and/or a component thereof.

The invention particularly relates to a medium or high voltageapparatus. The term “medium voltage” as used herein refers to a voltagein the range of 1 kV to 72 kV, whereas the term “high voltage” refers toa voltage of more than 72 kV. Applications in the low voltage rangebelow 1 kV are feasible, as well.

In order to set the respective parameters to the required value forachieving a desired breakdown field strength, the apparatus can comprisea control unit (also referred to as “fluid management system”) forcontrolling individually or in combination the composition—in particularthe chemical composition or the physical phase composition, such as agas/liquid two-phase system—and/or the temperature of the insulationmedium as well as the absolute pressure, the gas density, the partialpressure and/or the partial gas density of the insulation medium or atleast one of its components, respectively. In particular, the controlunit can comprise a heater and/or vaporizer in order to control thevapour pressure of the fluoroketone according to the invention. Thevaporizer can e.g. be an ultrasound vaporizer, or can comprise sprayingnozzles for spraying the insulation medium into the apparatus.

In an exemplary embodiment for high voltage applications, a partialpressure of the fluoroketone can be provided in the insulating medium byheating and/or vaporizing, such that the partial pressure offluoroketone is maintained at a pressure level of at least 0.6 bar ingas-insulated switchgears (GIS) busbars or gas-insulated transmissionlines (GITL), corresponding to conventional insulation distances (withapproximately required field strengths of about 300 kV/cm) andconventional pressure levels of e.g. about 4 bar. Accordingly, in ahigh-voltage circuit breaker the heating and/or vaporizing shall beadapted such that the partial pressure of the fluoroketone is maintainedat a pressure level of at least 0.9 bar, corresponding to conventionalinsulation distances (with approximately required field strengths ofabout 440 kV/cm) and conventional pressure levels of e.g. about 6 bar.

If a vaporizer is used, it usually also comprises a dosing unit to setthe concentration of the fluoroketone in the insulation medium accordingto needs of breakdown field strength. This will exemplarily be shown inmore detail below for a high voltage gas-insulated switchgear.Furthermore, the control unit may comprise a measuring unit formeasuring the control parameters, such as temperature, pressures and/orcomposition—in particular the liquid phase level—and/or a monitoringunit for monitoring such parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by way of the following Example incombination with the Figures of which

FIG. 1 a shows a graphical representation of the pressure reducedbreakdown field of an insulating medium according to the presentinvention as a function of its mole fraction ofdodecafluoro-2-methylpentan-3-one in comparison to the breakdown fieldof conventional insulation gases;

FIGS. 1 b-1 c show the absolute pressure of the insulation medium as afunction of the partial pressure of dodecafluoro-2-methylpentan-3-one

FIG. 2 shows a graphical representation of the vapour pressure ofdodecafluoro-2-methylpentan-3-one as a function of the temperature;

FIG. 3 a, 3 b, 3 c show for various concentration levels, i.e. molefractions, of dodecafluoro-2-methylpentan-3-one in air as carrier gasthe respective pressure and temperature values at which an exemplarybreakdown field strength of 440 kV/cm or 50 kV/cm is achieved;

FIG. 4 shows a purely schematic representation of a high voltagegas-insulated switchgear according to the present invention comprising atemperature control unit; and

FIG. 5 shows a purely schematic representation of a high voltagegas-insulated switchgear according to the present invention comprising afluid handling unit.

DETAILED DESCRIPTION OF THE INVENTION Examples

For measuring the breakdown field strength of an insulation mediumaccording to the present invention, a test vessel comprisingdodecafluoro-2-methylpentan-3-one (Novec 649, available from 3M) wasevacuated down to about 140 mbar and the pressure was successivelyincreased by adding ambient air as buffer gas up to about 5 bar. Forselected mole fractions of dodecafluoro-2-methylpentan-3-one in theresulting insulation gas, the breakdown field strength was determined ina pin-plate electrode arrangement under dc-voltage applied.

As shown in FIG. 1 a, the pressure-reduced breakdown field strength forthe insulation medium according to the present invention increaseslinearly as a function of an increasing mole fraction of the fluorketoneof the present invention, here selected to bedodecafluoro-2-methylpentan-3-one. At a mole fraction above 15%, theinsulation medium according to the present invention has a breakdownvoltage higher than the most conventional insulation gas according tothe state of the art.

FIGS. 1 b and 1 c show the absolute filling pressure of the insulationmedium according to the present invention as a function of the molefraction of the fluorketone of the present invention, here selected tobe dodecafluoro-2-methylpentan-3-one. FIG. 1 b and 1 c are obtained fromFIG. 1 a by choosing a permissible field strength of the electricalapparatus, by transforming the abscissa (y-axis) of FIG. 1 a by dividingthe values by the permissible field strength and inverting the resultingvalues to arrive at an absolute pressure scale and hence absolutepressure curve, and by multiplying the ordinate (x-axis) with theabsolute pressure curve to arrive at the partial pressure of thefluorketone of the invention, here preferably ofdodecafluoro-2-methylpentan-3-one. The permissible field strength ischosen to be exemplarily 440 kV/cm in FIG. 1 b and 50 kV/cm in FIG. 1 c.

In FIG. 2, the vapour pressure of dodecafluoro-2-methylpentan-3-one as afunction of the temperature is shown. The (absolute) pressure of theinsulating gas shall be chosen such that given the partial gas pressureof the fluoroketone (defined by the minimal operating temperatureaccording to FIG. 2) the desired breakdown field strength is obtained.

As well, an operating temperature can be determined for a givenbreakdown field strength and absolute pressure of the system. Forexample, a breakdown field strength of 440 kV/cm at an absolute pressureof 2.5 bar is according to FIG. 1 achieved at a molar ratio ofdodecafluoro-2-methylpentan-3-one of 0.5. The partial pressure ofdodecafluoro-2-methylpentan-3-one in the insulation gas is thus 1.25bar. According to FIG. 2, this partial pressure is obtained at atemperature of 56° C.

From FIG. 1 b or 1 c in combination with FIG. 2, a method for selectingthe parameters of the insulation medium, such as absolute fillingpressure, molar fraction or partial pressure of fluorketone and fluidmanagement, in particular heating and/or vaporization of liquid phasefluorketone, and/or fluid reserve management of liquid phasefluorketone, can be deduced.

This method comprises the steps of:

-   -   determining for a given electrical apparatus a permissible        electrical field strength of the desired insulation medium and a        minimal permissible operating temperature of the desired        insulation medium,    -   determining from the pressure-reduced breakdown field strength        of the desired insulation medium as a function of the molar        fraction of the fluorketone of the invention (see e.g. FIG. 1        a), hereinafter preferably with 6 to 9 C-atoms and more        preferably dodecafluoro-2-methylpentan-3-one, and from the        permissible field strength the absolute pressure curve of the        insulation medium as a function of the partial pressure of the        fluorketone (see e.g. FIG. 1 b or FIG. 1 c),    -   selecting a desired absolute filling pressure of the insulation        medium (which is typically defined for some standard conditions        and may, e.g., be based on constructive and/or operational        constraints of the electrical apparatus),    -   determining from the absolute pressure curve the minimal        required partial pressure of the fluorketone, and from the        vapour pressure curve the corresponding vaporization temperature        of the fluorketone, and    -   determining whether the vaporization temperature is above the        minimal permissible operating temperature of the desired        insulation medium, and    -   only if the vaporization temperature is below the minimal        permissible operating temperature of the desired insulation        medium, providing a fluid management system, in particular means        for heating and/or vaporization and/or fluid reserve management        of liquid phase fluorketone, for maintaining the partial        pressure above the minimal required partial pressure.

A further detailed example is shown in FIG. 1 c in connection with FIG.2 for a medium-voltage apparatus being rated to a given voltage level,from which the permissible electrical field strength of the desiredinsulation medium can be derived (e.g. 50 kV/cm), and being rated to anambient temperature, from which the minimal permissible operatingtemperature of the desired insulation medium can be derived (e.g. −25°C.). According to FIG. 2 extrapolated to −25° C., the partial pressureof fluorketone of the invention, here exemplarilydodecafluoro-2-methylpentan-3-one, at −25° C. is approximately 0.025bar, which according to FIG. 1 c requires approximately 0.95 barabsolute filling pressure. This is below the (e.g. apparatus-specific)permissible filling pressure of e.g. 1.2 bar, such that no activevaporization of liquid fluorketone is needed.

A further dimensioning rule relates to the maximal permissible operatingtemperature of the desired insulation medium, e.g. 105° C. inhigh-voltage or medium-voltage apparatuses. According to FIG. 2, 105° C.corresponds to a fluorketone partial pressure 5 bar, which may result inthe absolute pressure exceeding all permissible (e.g.apparatus-specific) pressure limits. This shall be avoided by limitingthe amount of available liquid fluorketone and/or limiting thetemperature, e.g. by active cooling. Therefore, in the apparatus areserve volume of liquid fluorketone and/or a maximal permissibleoperating temperature of the desired insulation medium shall be limitedsuch that the absolute filling pressure is maintained below a givenpressure limit of the apparatus (maximal permissible operatingpressure). The apparatus shall thus have a reserve volume of liquidfluorketone and/or means for limiting a maximal permissible operatingtemperature of the desired insulation medium such that the absolutefilling pressure is maintained below a given pressure limit of theapparatus.

FIGS. 3 a, 3 b and 3 c show further the relationship between theabsolute filling pressure and the temperature of the insulation gasrequired to obtain a given breakdown field strength (=permissibleelectrical field strength, here exemplarily 440 kV/cm and 50 kV/cm,respectively) for various molar fractions M of the fluorketone of theinvention. As is apparent, the dielectric field strength of theinsulation gas can be increased by increasing the molar ratio M of thefluoroketone, in this particular case ofdodecafluoro-2-methylpentan-3-one, and/or by increasing the total orabsolute filling pressure. In FIG. 3 a for example, a high-voltagebreakdown field strength of 440 kV/cm is achieved at a pressure of about7 bar and a temperature of about 22° C., the molar ratio of thefluoroketone being 5%. The same breakdown field strength is achieved ata pressure of less than 2 bar, but a temperature of 60° C., the molarratio of fluoroketone being 100%.

In FIG. 3 b for example, a medium-voltage breakdown field strength of 50kV/cm is achieved at an absolute filling pressure of about 0.8 bar and atemperature of about −20° C., the molar ratio of the fluoroketone being5%. The same breakdown field strength is achieved at a pressure of about0.1 bar and a temperature of about 5° C., the molar ratio M offluoroketone being 100%.

FIG. 3 c shows once more the admissible parameter range for the case ofa high-voltage breakdown field strength of 440 kV/cm. The horizontaldashed line between points 1 and 2 represents the apparatus-specificmaximal permissible absolute pressure, here e.g. 6 bar. The verticaldashed line between points 2 and 3 represents the maximal permissibleoperating temperature, here e.g. 105° C. The limiting absolute pressurecurve for molar ratio M=100% extends between points 4 and 3. Thedrawn-through curve between points 1 and 4 is the absolute pressurecurve as a function of temperature and of molar ratio of fluorketone ofthe invention, here e.g. dodecafluoro-2-methylpentan-3-one, as takenfrom FIG. 3 a. The encircled area, i.e. the area delimited by the linesconnecting in sequence the points 1-2-3-4-1, defines the range ofadmissible parameters, namely absolute filling pressures, operatingtemperatures of the desired insulation medium, and molar ratios (orcorrespondingly partial pressures) of the fluorketone of the inventionfor a selected breakdown field strength or permissible electrical fieldstrength.

As mentioned above, the electrical apparatus of the present inventioncan comprise a control unit (or “fluid management system”) in order toadapt the pressure, the composition and/or the temperature of theinsulating medium.

As an example, a high voltage switchgear comprising a temperaturecontrol unit is shown in FIG. 4. The switchgear 2 comprises a housing 4defining an insulating space 6 and an electrical active part 8 arrangedin the insulating space 6. The switchgear 2 further comprises atemperature control unit 10 a for setting the housing 4, or at least apart of the housing 4, of the switchgear and thus the insulation mediumcomprised in the insulating space 6 to a desired temperature. Of course,any other part in contact with the insulation medium can be heated inorder to bring the insulation medium to the desired temperature. Thus,the vapour pressure of the fluoroketone—and consequently its molar ratioin the insulation gas—as well as the absolute pressure of the insulationgas can be adapted accordingly. As also shown in FIG. 4, thefluoroketone is in this embodiment not homogenously distributedthroughout the insulating space due to the temperature gradient given inthe insulation space. The concentration of the fluoroketone is thushigher in close proximity to the walls 4′ of the housing 4.

An alternative control unit or fluid management system is schematicallyshown in FIG. 5 in which a fluid handling unit 10 b is attributed to thegas-insulated switchgear as the control unit. According to this controlunit, the composition of the insulating medium, and in particular itsconcentration of the fluoroketone, is adjusted in a respective dosingunit comprised in the fluid handling unit 10 b, and the resultinginsulation medium is injected or introduced, in particular sprayed, intothe insulating space 6. In the embodiment shown in FIG. 5, theinsulation medium is sprayed into the insulating space in the form of anaerosol 14 in which small droplets of liquid fluoroketone are dispersedin the respective carrier gas. The aerosol 14 is sprayed into theinsulating space 6 by means of nozzles 16 and the fluoroketone isreadily evaporated, thus resulting in an insulating space 6 with aninhomogenous concentration of fluoroketone, specifically a relativelyhigh concentration in close proximity of the housing wall 4′ comprisingthe nozzles 16. Alternatively, the insulation medium, in particular itsconcentration, pressure and temperature, can be controlled in the fluidhandling unit 10 b before being injected into the insulation space. Inorder to ensure circulation of the gas, further openings 18 are providedin the upper wall 4″ of the housing 4, said openings leading to achannel 20 in the housing 4 and allowing the insulating medium to beremoved from the insulating space 6. The switchgear with fluid handlingunit 10 b, as shown in FIG. 5, can be combined with the temperaturecontrol unit 10 a described in connection with FIG. 4. If no temperaturecontrol unit is provided, condensation of the fluoroketone can occur.The condensed fluoroketone can be collected and reintroduced into thecirculation of the insulation medium.

In the context of the switchgears shown in FIGS. 4 and 5 it is notedthat nominal current load generally facilitates the vaporization of thefluoroketone by the ohmic heating of current carrying conductors.

What is claimed is:
 1. A dielectric insulation medium comprising aninsulation gas, said insulation gas comprising at operational conditionsa fluoroketone having from 4 to 12 carbon atoms, characterized in thatthe fluoroketone has a boiling point of at least −5° C. at ambientpressure.
 2. The insulation medium according to claim 1, characterizedin that the fluoroketone has the general structureR1—CO—R2 wherein R1 and R2 are at least partially fluorinated chains,said chains being independently from each other linear or branched andhaving from 1 to 10 carbon atoms.
 3. The insulation medium according toclaim 1, characterized in that the fluoroketone has from 4 to 10 carbonatoms.
 4. The insulation medium according to claim 1, characterized inthat the fluoroketone has from 4 to 8 carbon atoms.
 5. The insulationmedium according to claim 3, characterized in that the fluoroketone has6 carbon atoms.
 6. The insulation medium according to claim 5,characterized in that the fluoroketone isdodecafluoro-2-methylpentan-3-one.
 7. The insulation medium according toclaim 1, characterized in that the molar ratio of the fluoroketone inthe insulation gas is at least 1% or at least 2%.
 8. The insulationmedium according to claim 7, characterized in that the molar ratio ofthe fluoroketone in the insulation gas is at least 5% or at least 10° orat least 15%.
 9. The insulation medium according to claim 1,characterized in that the insulation gas is a gas mixture, which furthercomprises air or at least one air component.
 10. The insulation mediumaccording to claim 9, characterized in that the insulation gas is a gasmixtures which comprises at least one air component selected from thegroup consisting of carbon dioxide, oxygen and nitrogen.
 11. Adielectric insulation medium comprising an insulation gas, saidinsulation gas comprising at operational conditions a fluoroketonehaving from 4 to 12 carbon atoms and having the general structureR1—CO—R2 wherein R1 and R2 are at least partially fluorinated chains,said chains being independently from each other linear or branched andhaving from 1 to 10 carbon atoms.
 12. The insulation medium according toclaim 11, characterized in that the fluoroketone has a boiling point ofat least −5° C. at ambient pressure.
 13. The insulation medium accordingto claim 11, characterized in that the fluoroketone has from 4 to 10carbon atoms.
 14. The insulation medium according to claim 11,characterized in that the fluoroketone has from 4 to 8 carbon atoms. 15.The insulation medium according to claim 11, characterized in that thefluoroketone has 6 carbon atoms.
 16. The insulation medium according toclaim 11, characterized in that the molar ratio of the fluoroketone inthe insulation gas is at least 1% or at least 2%.
 17. The insulationmedium according to claim 16, characterized in that the molar ratio ofthe fluoroketone in the insulation gas is at least 5% or at least 10% orat least 15%.
 18. The insulation medium according to claim 11,characterized in that the insulation gas is a gas mixture, which furthercomprises air or at least one air component
 19. The insulation mediumaccording to claim 18, characterized in that the insulation gas is a gasmixtures which comprises at least one air component selected from thegroup consisting of carbon dioxide, oxygen and nitrogen.
 20. Adielectric insulation medium comprising an insulation gas, saidinsulation gas comprising at operational conditions a fluoroketone,characterized in that the fluoroketone has 6 carbon atoms.
 21. Theinsulation medium according to claim 20, characterized in that thefluoroketone has the general structureR1—CO—R2 wherein R1 and R2 are at least partially fluorinated chains,said chains being independently from each other linear or branched andhaving from 1 to 10 carbon atoms.
 22. The insulation medium according toclaim 20, characterized in that the fluoroketone has a boiling point ofat least −5° C. at ambient pressure.
 23. The insulation medium accordingto claim 20, characterized in that the fluoroketone is a perfluorinatedketone having the molecular formula C₆F₁₂O.
 24. The insulation mediumaccording to claim 23, characterized in that the fluoroketone isdodecafluoro-2-methylpentan-3-one.
 25. The insulation medium accordingto claim 20, characterized in that the molar ratio of the fluoroketonein the insulation gas is at least 1% or at least 2%.
 26. The insulationmedium according to claim 25, characterized in that the molar ratio ofthe fluoroketone in the insulation gas is at least 5% or at least 10% orat least 15%.
 27. The insulation medium according to claim 20,characterized in that the insulation gas is a gas mixture, which furthercomprises air or at least one air component.
 28. The insulation mediumaccording to claim 27, characterized in that the insulation gas is a gasmixtures which comprises at least one air component selected from thegroup consisting of carbon dioxide, oxygen and nitrogen.